Adaptive characteristics of leaf phenotypic plasticity and integration in Calamagrostis angustifolia under long-term nitrogen addition.

Based on a long-term nitrogen (N) addition experiment in the Sanjiang Plain wetland, we measured leaf photosynthetic physiology, morphological structure, and ecological stoichiometric characteristics of the dominant species Calamagrostis angustifolia among three treatments: control (0 g N·m-2·a-1), low N (4 g N·m-2·a-1), and high N (8 g N·m-2·a-1). We elucidated the response patterns and adaptive strategies of its functional traits after 14 years chronic N inputs. The results showed that 14 leaf functional traits responded differently to long-term N addition. Under the low N treatment, leaf nitrogen content, specific leaf area (SLA), and net photosynthetic rate increased significantly by 199.9%, 45.7%, and 27.1%, respectively, indicating an efficient acquisitive strategy of "high-investment and high-return". Under the high N treatment, leaf nitrogen content and SLA significantly increased relative to the control, whereas the net photosynthetic rate decreased significantly compared to the low N treatment, and photosynthetic nitrogen-use efficiency (PNUE) dropped by 76.8% compared with the control, indicating an inefficient acquisitive strategy of "investment-return imbalance". These changes in leaf functional traits indicated that long-term N addition drove a shift in resource-use strategy of C. angustifolia from conservative to acquisitive. There was no difference in overall phenotypic plasticity between the low and high N treatments, but the response magnitudes of the 14 functional traits varied. Leaf nitrogen content and leaf N:P exhibited the strongest plasticity. The plasticity of net photosynthetic rate, stomatal conductance, transpiration rate, PNUE, and water use efficiency (WUE) was higher under the low N treatment. Phenotypic integration under high N treatment was significantly higher than under the low N treatment, with tighter correlations among leaf functional traits. Phenotypic plasticity and phenotypic integration showed no correlation, suggesting that they acted as relatively independent dimensions, playing roles in optimizing resource utilization and coping with environmental stress, respectively.